The use of powder metallurgical techniques in the production of metal parts is well established. In such manufacturing, iron or steel powders are often mixed with one other alloying element, also in particulate form, followed by compaction and sintering. The presence of the alloying elements permits the attainment of strength and other mechanical properties in the sintered parts at levels that could not be reached with unalloyed iron or steel powders alone.
In one aspect of this alloying procedure, it is an aim to have additional metals adhered in some manner to the surface of the iron-based particles so that upon compaction and sintering, desired alloys form along the grain boundaries. One art-recognized technique for accomplishing this result is to coat the iron-based particles with a sticky substance and then apply a dusting of the alloying materials, in fine particulate form, to coat the iron-based particles. The coated iron-based particles can then be heated to produce diffusion-bonded alloy particles on the surface of the core particles. The final parts made from the compaction and sintering of such pretreated powders have been known to attain improved density and strength. However, the original application of the alloying metal to the surfaces of the individual iron particles is often not uniform.
In some practices, the iron-based particles are admixed with particles of the alloying material as well as with small amounts of an organic binder that is used to bind or "glue" the alloying powders to the iron-based particles. Such compositions are generally not subjected to a pretreatment in order to diffusion-bond the alloying particles to the surfaces of the underlying iron-based particles, but rather are used "as is" in the further compaction and sintering steps leading to the finished metal part. It is known, however, that some such organic binders have adversely affected the compressibility of the powder, thereby lowering the density of the pressed "green" part as well as that of the final sintered part.
Powder metallurgical compositions are also traditionally provided with a lubricant, such as a metal stearate, a paraffin, or a synthetic wax, in order to facilitate ejection of the compacted green component from the die. The friction forces which must be overcome in order to remove a compacted part from the die, which generally increase with the pressure used to compact the part, are measured as the "stripping" and "sliding" pressures. The lubricants generally reduce these pressures, but their presence can also adversely affect compressibility of the powder composition. Although the compressibility of iron-based powder compositions that contain particulate alloy materials can be increased by reducing the amount of lubricant used, the resulting decrease in lubricity can cause unacceptably large increases in the ejection forces, which can result in scoring of the die, loss of die life, and imperfections in the surface of the compacted part. A traditional method for combining a lubricant with a metallurgical powder is to combined the lubricant, generally in solid particulate form, with the metal powder itself.